Recently, low input voltage (VIN) charge pumps for low voltage and low power electronics device applications were composed with direct current (DC)-to-DC converters using a large inductor. In some conventional approaches, charge pumps use an advanced process technology to push the input voltage down to the sub-threshold region. One conventional approach for a 3-stage voltage doubler uses a forward body biasing (FBB) technique, which improves the voltage conversion efficiency (VCE) for low input voltages. However, this approach suffers from poor power conversion efficiency. Also, in another conventional approach, the charge pump can use the forward body bias to regulate the output voltage of a charge pump to the reference voltage. In yet another conventional approach, a charge pump can be implemented using a 10-stage design, which achieves a low operation voltage. However, a 10-stage charge pump design provides low output power.
Most capacitive voltage multipliers used to generate a DC voltage higher than the supply voltage are based on the Dickson charge pump, which takes a DC supply as its input, so it is a form of DC-to-DC converter. Typically, the Dickson multiplier is intended for a low voltage purpose.
Some conventional charge pump cells include cross-coupled NMOS transistors and pumping capacitors. When a charge pump cell uses NMOS transistors, two outputs of the cell are connected together to a single output via PMOS transistors, which is called a dual series switch or load switches, to avoid a large threshold voltage (VTH) drop for low voltage applications.
There are two kinds of methods to drive load switches. FIG. 1 shows a voltage doubler with cross-coupled load switches, according to the prior art. The load switches are self-driven by pumped input signals CT and CTB. This type of voltage doubler is called “CP-cross.”
FIG. 2 shows a voltage doubler with load switches driven by a level shifter (LS), according to the prior art. A voltage doubler with load switches driven by a level shifter improves the conductance of the dual series switches. This type of voltage doubler is called “CP-LS.” However, prior arts such as CP-cross and CP-LS show poor performance at low input voltages.
Usually, the body of NMOS transistors is applied to the most negative voltage in the circuit design; whereas, the body of PMOS transistors is applied to the most positive voltage. The conventional CP-cross can be improved by using a bulk switching (BS) technique, as shown in FIG. 3. FIG. 4 shows a detailed switching operation of the circuit shown in FIG. 3. In FIG. 3, the NMOS switches MN1 and MN2 always provides the bulk-to-source PN junction in the reverse bias mode. This means that low leakage current can be guaranteed during the off state of the NMOS switches MN1 and MN2. At the same time, the use of NMOS is a better choice than using PMOS because of higher mobility. The BS technique keeps the body of the PMOS switches at the highest voltage, even though the load current is heavy so that the voltage drop at the output is large, which avoids degradation of the threshold voltage due to the body effect.
However, the conventional biasing approach is not effective at low input voltage. As the input voltage reaches VTH,N+VTH,P, where VTH,N is an NMOS threshold voltage and VTH,P is a PMOS threshold voltage, reverse body biasing (RBB) for NMOS prevents NMOS switches from being turned on. Therefore, the forward body biasing (FBB) for low voltage operations was introduced in some approaches. In these approaches, higher current transfer via switches was considered to be more important than higher leakage current at low input voltages. FIG. 5 is a forward body biasing configuration for a voltage doubler, according to the prior art. As shown in FIG. 5, all MOSFETs are biased in the forward mode. FIG. 6 shows a detailed switching operation of the circuit shown in FIG. 5. In this biasing scheme, the NMOS transistors should be in deep n-well. The forward body biased switches present high on-current. The parasitic path of a lateral bipolar also contributes to delivering more current to the output. Nevertheless, high leakage current during off-state of switches degrades the power efficiency of such a voltage doubler.
Accordingly, there remains a need in the art for a charge pumping apparatus that overcomes the limitations of conventional approaches.